What have you done so far?
If the answer is nothing, you need to start with a site audit to find out what needs to be done such as find broken links, check meta tags, page load speeds, check crawl errors etc.
Some of the most important SEO basics are as follows:
Install and configure SEO plugin, create meta titles, meta descriptions for existing content, create site map, notify search engines so they can crawl and index content.
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